The present invention relates to equipment of the type used in surveying and construction and, more particularly, to a detector device for such applications which has an improved photo-detector arrangement for detecting the position of reference light. The light may typically be a rotating laser beam that defines a reference plane of light or a stationary reference plane or cone of laser light. The reference plane may be horizontal or inclined, as dictated by the application.
Laser systems have commonly been employed in surveying and construction in which a laser beam is rotated in either a horizontal or a graded plane. U.S. Pat. No. 4,062,634, issued Dec. 13, 1977, to Rando, illustrates a laser transmitter that provides such a rotating reference beam. The rotating beam defines a reference plane from which various measurements can be made. For example, the elevation of a point remote from the laser transmitter may be measured using a rod on which a laser receiver is mounted. The bottom of the rod rests on the ground, and the operator adjusts the receiver along the rod to a position where it intercepts the laser beam, as indicated by a display on the receiver. One such laser receiver is shown in U.S. Pat. No. 4,240,208, issued Jun. 30, 1987, to Pehrson. Another laser receiver is shown in published U.S. patent application Ser. No. 09/965,661, filed Sep. 27, 2001 and published Mar. 27, 2003 as Publication No. 2003/0058446A1.
Laser receivers operating at construction sites may be illuminated by a number of extraneous light sources that must be ignored or at least distinguished from the laser beam. For example, some types of laser transmitters provide an indication of the azimuth angle of the laser beam relative to a reference direction by transmitting a strobe pulse of light once during each beam rotation. Further, it is common for strobe lights to be used as warning lights at a construction site where machine control and surveying operations are taking place.
It will be appreciated, therefore, that a laser receiver operating in such an environment must be able to distinguish between illumination by a laser beam and illumination by a pulse of light from a strobe source. One way to distinguish between the two is to provide separate detector elements for the laser light and the strobe light, each with optical filters to screen out light except for the frequencies of interest. This may not always be completely effective, however, and in addition to the unwanted interference, the optical filters may also attenuate light to some degree at all frequencies, reducing the sensitivity of the receiver.
It is seen, therefore, that there is a need for a device and method for detecting light from a laser transmitter and from strobe sources, and for distinguishing between the two.